Microsoft has released its security patches for September, fixing known vulnerabilities in its MSN Messenger software and Unix services for Windows as well as a critical bug in Windows 2000. In total, Microsoft patched four bugs in its products. This was one less update than expected following the software maker's last-minute decision to scrap a fix for its SharePoint collaboration software.

The Windows 2000 update is the only one Microsoft rates as critical. It affects the Microsoft Agent software that Web developers use to create interactive characters on Web pages. By tricking a Windows 2000 user into visiting a maliciously encoded Web page, an attacker could exploit this flaw to run unauthorized software on a victim's computer.

This is not the first time Microsoft has been forced to patch a critical bug in Microsoft Agent. In April, the company fixed a similar flaw that also affected Windows XP users. Because the MSN Messenger and Windows Services for Unix flaws were both publicly disclosed last month, they should also be given priority said Amol Sarwate, manager of Qualys's vulnerability research lab.